In a control system including a single control unit and a plurality of controlled devices (each including a controlled part that is operable in response to a command from the control unit and a sensor unit that transmits information to the control unit), reduction of the number of lines, that is, so-called “line reduction” has widely been performed. As a general method of achieving the line reduction, a method has widely been adopted, in which, instead of parallel connection for directly connecting each of signal lines extending from the controlled devices to the control unit, a main station and a plurality of substations are both provided with a converting function of a parallel signal and a serial signal, and connected to the control unit and the controlled devices, respectively, so that transmission and reception of data is performed by the serial signal between a main station and the substations.
When the line reduction is achieved, in a state where many substations are subjected to connection, if the control unit side can not identify a location where disconnection has occurred, there are needs of checking each of the lines of the substations located far away from the control unit, and a considerable amount of man-hours is required to identify the disconnected location.
To deal with this problem, there has been proposed a method in which disconnection on the substation side is checked in the control unit side. For example, a method of detecting disconnection in a time-division-multiplexing transmission system has been disclosed in Japanese Patent Application Laid-open No. S62-173830, in which a plurality of terminal units assigned with respective addresses is connected to a single master unit via a common signal line, a transmission signal is sent to the terminal units from the master unit while sequentially changing a designated address in a cyclic manner, and a reply signal is sent to the master unit from the addressed terminal unit. In this method of detecting disconnection, it is determined that there is disconnection on the signal line when the master unit detects a state in which the reply signal can not be received from the terminal unit occurring continuously for a period of the predetermined number of transmission cycles.
In this method of detecting disconnection, a command system is employed as a system of performing transmission and reception of data between the devices, in which a counterpart (substation) for performing the transmission and reception of data is designated by data (address data) for identifying the counterpart. Even in the method of performing the transmission and reception of data by the serial signal in the case of line reduction, it is possible to employ this command system. However, in the command system, the transmission efficiency is degraded if the command data (address data) is significant relative to the amount of data to be transmitted. Then, in order to increase the transmission efficiency with the serial signal, a transmission synchronization system may be preferred in which the transmission and reception of data is performed using a monitoring signal and a control signal that are sequentially transmitted in a pulse cycle that is not involved in the address data. In this manner, the command system and the transmission synchronization system are used differently depending on the usage in the line reduction technique, and even in a system employing the transmission synchronization system, there has been proposed a method in which a control unit side checks line disconnection of the substation side.
For example, in Japanese Patent Application Laid-open No. H7-99503, a data transmission apparatus has been disclosed, which can easily identify a location where an abnormality has occurred in a transmission line. In this data transmission apparatus, an answer unit is provided in each terminal unit, wherein the answer unit outputs a confirmation signal in the last area of one round of a pulse signal transmitted on a signal line under a condition that a count value of a round counter of its own station reaches a predetermined value assigned to the station. Then, an abnormality detection unit connected to the signal line determines the presence or absence of the confirmation signal from the answer unit of the terminal unit corresponding to the count value of its own round counter. It is then determined that there is no connection abnormality in the terminal unit corresponding to the count value when there is the confirmation signal, and a check signal is outputted to the last area of one round of the pulse signal to be transmitted on the signal line. On the other hand, when there is no confirmation signal, it is determined that there is a connection abnormality in the terminal unit corresponding to the count value, and no check signal is outputted, by which the master unit can detect disconnection of line.
Furthermore, Japanese Patent Application Laid-open No. H9-84155 discloses a technique in which, in a control and monitoring system that includes a single master station and at least one slave station connected to the master station in a branched manner, and performs control with transmitting a control signal sent from the master station to the sub-station while transmitting a monitoring signal including a state of the slave station to the master station, the master station that is situated at the center can detect a location where an abnormality has occurred, line disconnection is detected even if a transmission cable is branched, and states of a controlled part, a sensor unit, and the like are identified at the center. In the system disclosed in this literature, a frame sent from the master station is composed of a start signal having a long period of a power-source voltage level, a data signal, and signal and end signal of an identification code (ID code) of a slave station, and an end signal, and a frame with a sequentially changed ID code is sent out in a sequential manner. In each slave station, an ID code assigned to its own station and a count value indicating a data transmission and reception address are set, and an end response is generated at a timing of the end signal when the former ID matches the ID code received from the master station. In the master station, the end response from the corresponding slave station is stored, thus storing a configuration of the slave stations.